No idea on the Castor oil....never heard of anyone using it. I don't even know if it's water soluble. You can pick up a bottle of Murphy's pretty cheap, and mixed up it will probably last you for years. I've never heard of it leaving a residue before. It is all water soluble, so there is nothing to create problems as far as I am concerned.
There are lots of things out there to accomplish the same task....this is just one that has worked well for me.
There are lots of things out there to accomplish the same task....this is just one that has worked well for me.